1. Field of the Invention
The present invention relates to a device for estimating information on a target object existing around a vehicle by radiating a radar wave and receiving the wave reflected from the object.
2. Description of Related Art
A millimeter wave radar apparatus located in a controlled vehicle has been used to detect information on a target object existing around the vehicle. This radar apparatus radiates a beam of radar wave set in the millimeter wave band as a high-frequency signal while mechanically or electrically scanning the wave in the horizontal direction (i.e., a width direction of the vehicle) and detects, from the wave reflected from the object, a distance from the vehicle to the target object, a relative speed of the target object to the vehicle, a bearing angle from the vehicle to the target object and the like as information on the target object. The beam of the radar wave has a predetermined width in the vertical direction (i.e., a height direction of the vehicle).
This radar apparatus cannot detect the height of the object (i.e., information on the object in the height direction). Therefore, it is difficult for the radar apparatus to distinguish an unnecessary reflecting object having no possibility of collision with the controlled vehicle from an obstacle (e.g., an object dropped on the road or a forward vehicle running in front of the controlled vehicle) having a probability of collision with the controlled vehicle. Because the vehicle can pass over a lid of a man hole located on the road or can pass under a signboard located over the road, the lid and the signboard denote the unnecessary reflecting objects.
To solve this problem, there is an idea that the beam width of the radar wave in the vertical direction is narrowed to narrow the information detecting range in the vertical direction. In this idea, the radar apparatus hardly detects the unnecessary reflecting object. However, when the vehicle is placed at a point at which the road gradient (i.e., inclination of the road in the vertical direction) is changed, the detecting distance of the apparatus is reduced, and the apparatus cannot detect the obstacle which is away from the vehicle by a distance longer than the reduced detecting distance. Further, as the change in the road gradient is increased (i.e., as the radius of a crossing curve is reduced), the reduction of the detecting distance is enlarged. Therefore, it is improper to excessively narrow the beam width in the vertical direction.
The radar wave reflected from a target object returns to the radar apparatus through a plurality of transmission paths, and the reflected waves transmitted through the respective paths interfere with each other in the radar apparatus. In this case, the electric power of the reflected waves received in the radar apparatus is changed with the distance to the reflecting object due to the multipath phenomenon.
Published Japanese Patent First Publication No. H05-196725 proposes an idea for receiving radar waves reflected at a reflecting point and returned to a radar apparatus through respective transmission paths and detecting the height of the reflecting point from a change in the electric power of the received waves changed with the distance due to the multipath phenomenon. When the height of a reflecting point (i.e., the height of a locating position at which a target object is located to be placed over the road or the height of a target object located on the road) reflecting the radar wave is higher than the road surface, the multipath phenomenon occurs in the received radar waves. In this case, a phase difference based on a path length difference occurs between the reflected wave directly returned from the target object to the apparatus and the reflected wave returned from the target while being once reflected on the road surface. When the phases of the received waves differ from each other approximate π radians, these received waves almost cancel out at the wave receiving point of the apparatus, and the electric power of the received waves is considerably reduced.
The reduction of the received electric power will be described with reference to FIG. 1. FIG. 1 is an explanatory view showing a typical model of reflected waves returned from a target object to a radar apparatus through respective paths. The radar apparatus is located on a controlled vehicle Vc and stores the height h of a wave transmitting and receiving point A. As shown in FIG. 1, a reflecting point B of the target object is placed at a height H and is spaced from the receiving point A of the radar apparatus by a distance R in the horizontal direction. One reflected wave is directly returned to the apparatus through a transmission path having a transmission distance RAB, while another reflected wave is returned to the apparatus through another transmission path having a transmission distance RACB while being once reflected on the road surface at a road reflecting point C. When the apparatus receives the reflected waves at the receiving point A having a known height h, the received waves interfere with each other. The phase difference between the received waves at the receiving point A is changed with the difference RACB-RAB, and this difference is determined from the height H of the target object B and the distance R. Therefore, the electric power P of the received waves is determined from the height H and the distance R according to a function f indicated by the equation (1).P=f(H,R)  (1)Therefore, the height H is expressed according to a function g indicated by the equation (2).H=g(R,P)  (2)
When the phases of the received waves having the path difference RACB-RAB differ from each other approximately by π radians, the power P is reduced to a minimal value Pnull, and the distance R is equal to a specific value (hereinafter, called a null distance) Rnull. Therefore, when the apparatus detects the power equal to the minimal value Pnull while detecting the null distance Rnull, the height H (H=g (Rnull, Pnull)) can be determined.
Accordingly, it is possible to detect the height of each target object with comparatively high precision. However, the computing load required to detect heights of many target objects becomes large. Therefore, a processor having a high throughput is inevitably required.
Published Japanese Patent First Publication No. 2004-198438 proposes an idea for judging, from a change in the electric power of the received waves, whether or not the multipath phenomenon is caused in the received waves. When the electric power of the received waves is changed, because a target object has a height so as to cause the multipath phenomenon, it is judged that the probability that a vehicle can pass over the target object is low. In contrast, when the electric power of the received waves is not changed, because a target object has a height so as not to cause the multipath phenomenon, it is judged that the probability that a vehicle can pass over the target object is high.
Therefore, even when a processor having a comparatively low throughput is used for the detection of target objects, it is possible to judge whether or not a vehicle holding the radar apparatus can pass over each target object. However, it is impossible to distinguish an obstacle, which is placed on the road and has a probability of collision with the vehicle, from an unnecessary reflecting object (e.g., a signboard or a traffic signal located over the road at a high position) under which the vehicle can pass.